The modern gas turbine engine is subject to both environmental and efficiency pressures. The engine must produce no or minimal levels of environmental pollutants such as NOx (oxides of nitrogen), CO (carbon monoxide), UHC (unburnt hydrocarbons) and smoke.
CO and UHC are produced as a cause of combustion inefficiency, whilst NOx and smoke emissions are caused by high temperatures and a slightly weaker than stoichiometric fuel to air ratio and richer than stoichiometric fuel to air ratio respectively.
In a lean-burn combustor, the flow of air into the combustor is increased such that the fuel to air ratio is below the level at which NOx is formed. The addition of extra air has the added effect of reducing the localised temperature of the gases formed by the combusted fuel, similarly minimising the chance for NOx to be formed.
One problem with lean-burn combustors is that the increased airflow can cause instability in the combustion process that results in high fluctuating pressure amplitudes at a frequency below 1000 Hz, and more particularly in the region of 600 to 800 Hz. The high fluctuating pressure amplitudes can cause hardware damage to the combustion chamber itself.
Combustion chambers may be cooled by a flow of air into the chamber through perforations in the wall of the chamber. The injected air, from holes commonly known as effusion holes, forms a film of relatively cold air over the inner surface of the combustor and reduces the value of the convective heat transfer between the flame and the combustor wall. The film must be uniform to prevent localised hot-spots and to ensure that the temperature of the wall is below the melting point of the material from which it is manufactured.
It has been proposed that the flow of air through the effusion holes may also be used to provide damping of instabilities in the combustion process.
The amount of air flowing through a turbine engine is limited and, where a lean burn combustor is provided, the additional air used in the combustion process constrains the amount of air available for damping and cooling purposes. Additionally, a flow of air providing a cooling function has different characteristics to a flow of air providing a damping function. Cooling air is injected at a spacing, flow volume and velocity that will not damp the pressure fluctuations. Similarly, damping air is necessarily injected at a spacing, flow volume and velocity that will not sufficiently cool the combustor walls.
The surface area of the damper is preferably kept as small as possible to minimise the area lost to cooling and to prevent the area from overheating.
It has been proposed to use Helmholtz resonators to provide damping of acoustic fluctuations and to damp high frequency oscillations, above 2000 Hz, such a device may be used. However, if it is required to damp low frequency oscillations, below 1000 Hz, such a resonator may not be feasibly be used in a gas turbine engine. The size of the resonator required to generate the Helmholtz resonance is inversely proportional to the frequency that it is desired to damp. Consequently, to damp low frequencies, a resonator chamber may be required of a size greater than that of the combustor chamber within which the frequencies are generated. Such a resonator is clearly impractical.